How can I pass std::unique_ptr into a function

Question

How can I pass a std::unique_ptr into a function? Lets say I have the following class:

class A
{
public:
    A(int val)
    {
        _val = val         


        

Answer 1

Why can I not pass a unique_ptr into a function?

You cannot do that because unique_ptr has a move constructor but not a copy constructor. According to the standard, when a move constructor is defined but a copy constructor is not defined, the copy constructor is deleted.

12.8 Copying and moving class objects

...

7 If the class definition does not explicitly declare a copy constructor, one is declared implicitly. If the class definition declares a move constructor or move assignment operator, the implicitly declared copy constructor is defined as deleted;

You can pass the unique_ptr to the function by using:

void MyFunc(std::unique_ptr& arg)
{
    cout << arg->GetVal() << endl;
}

and use it like you have:

or

void MyFunc(std::unique_ptr arg)
{
    cout << arg->GetVal() << endl;
}

and use it like:

std::unique_ptr ptr = std::unique_ptr(new A(1234));
MyFunc(std::move(ptr));

Important Note

Please note that if you use the second method, ptr does not have ownership of the pointer after the call to std::move(ptr) returns.

void MyFunc(std::unique_ptr&& arg) would have the same effect as void MyFunc(std::unique_ptr& arg) since both are references.

In the first case, ptr still has ownership of the pointer after the call to MyFunc.